Natural substances containing hydroxamic acid functionalities in their structures exhibit a wide variety of biological activities. Frequently, they act in vivo as antibiotics, growth factors, and iron-transport agents. Siderophores represent one type of natural substance that contain hydroxamic acid functionalities. Siderophores are small iron-chelating molecules produced by the organism when iron deficiencies occur. There are mainly two classes of siderophores, the cathecolamides and the hydroxamates. The cathecolamide class includes, for example, parabactin and vibriobactin. The hydroxamate class includes, for example, bisucaberin (macrocyclic), desferrioxamine B (linear), desferrioxamine G (linear) and desferrioxamine E (macrocyclic). Because of their numerous potential applications, such as in the treatment of iron overload, also known as Cooley's anemia, as contrast agents for NMR imaging, and as diagnostic and therapeutic radiopharmaceuticals, the design and synthesis of new chelators possessing hydroxamic acid residues have attracted the attention of several research groups.